Remove unnecessary scroll bar on sub menus

Setting overflow-x:scroll forces a scroll bar to be shown.

On the sub-menu, use overflow-x:auto always so the browser handles
deciding when a scroll bar should be drawn.

No new tests, covered by existing unit tests.

Bug: 1001481
Change-Id: Icf84c0ea9a40b65383bcb7be23be84d2f0ee3a6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1933858Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718576}

ui/file_manager/file_manager/foreground/js/ui/multi_menu.js

@@ -177,12 +177,8 @@ cr.define('cr.ui', () => {
       // allows the menu height to grow crbug/934207
       style.maxHeight =
           (viewportHeight - itemRect.top - this.menuEndGap_) + 'px';
-      if ((itemRect.top + childRect.height + this.menuEndGap_) >
-          viewportHeight) {
-        style.overflowY = 'scroll';
-      } else {
-        style.overflowY = 'auto';
-      }
+      // Let the browser deal with scroll bar generation.
+      style.overflowY = 'auto';
     }
 
     /**